On Sun, Mar 11, 2018 at 7:24 PM, Warin <61sundow...@gmail.com> wrote:
[in reply to my message:]
>> I'm sure that this difficulty is part of what motivates the OSM-Carto
>> group to be requesting that all ways that participate in admin
>> boundaries be tagged with the "most important" boundary in which they
>> appear. This tagging gives an easy route to displaying the border
>> unambiguously - drive the renderer from the ways and not the
>> relations.
> No. If I were to make a decision on which is 'more important' I will make it
> based on my interpretation. That may not match yours.

Right! We're in 'violent agreement' here.  I was describing the likely
motivation for wanting the tagging on the enclosing ways.
> The renders should make the choice based on what they want to render i.e.
> which one is 'more important' for that map.

Right again. Admin boundaries, however, are somewhat
heterogeneous. Many renderers want to show them - political
boundaries are important in a lot of contexts - and I don't
mind having an 'admin_level' hierarchy. That said, if a renderer
wants to impose a different idea of the world, it should be free to.

>> I think this is a particular case where the political problem needs a
>> technical solution. I have not yet been clever enough to invent a
>> method in the PostGIS-Mapnik pipeline to identify ways that
>> participate in administrative boundary relations, select the "most
>> important" boundary and display each way only once, no matter how many
>> relations use it.  This is, of course, what is needed for a good many
>> rendering problems, where the definition of "most important" may be
>> specific to a particular renderer. If we had skeleton code showing how
>> such a thing would operate, I'm sure that the OSM-Carto team could
>> adapt to it, (and so could I, on the maps that I render!).
> Yes.

I'm glad we're in agreement here. This was my key point: the fact
that the existing rendering technology makes implementing
'render at most one boundary on any given way' insanely
difficult when the boundaries are associated with relations
that the way is a member of. With osm2pgsql, I'm not sure
that's even possible without digging through the slim tables,
and I've certainly never managed to implement a rendering
that gets it right.

If we can fix the main technical issue, the tagging problem
becomes much less significant. I was hoping this discussion
might bring out of the woodwork a programmer who's clever
enough to solve it cleanly.

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